Power Transformer: The Ultimate FAQ Guide

Power Transformer have been around for more than a century and as time continues to move forward, so does the advancement of transformers. In fact, the use of transformers continues to increase as the world’s population grows.

Power transformers are a type of transformer that is currently in high demand all over the world because of their many useful functions and attributes, like supplying electricity to multiple houses, regulating voltages, etc.

In this article, one of the greatest transformer manufacturers in China – DAELIM, will provide you with all the necessary information you need to completely understand what power transformers are.

But first, you should know what transformers are in general.

Power Transformer

200KV Class Three Phase Two Winding OLTC/NLTC

Power Transformer

110KV Three Phase Three Winding OLTC/NLTC

Power Transformer

66KV Class Three Phase Two Winding OLTC/NLTC

Table of Contents

What is Transformer?

Transformers come in various sizes and shapes. There are transformers that are big as cars, and transformers that are the size of a barrel. Cooling mediums for transformers are different too.

Some transformers use pressurized air or forced air as a natural way of cooling or regulating the transformer’s temperature. Other transformers use oil as an effective way to keep temperature at moderate levels.

But regardless of the shape, size, and cooling medium, all transformers have one thing in common and that is to regulate voltages, current, or electricity in general.

Nevertheless, they are certainly not limited to that function. Transformers are used in many ways. An example of this is that it can be used to reduce voltage of conventional power circuits to operate low-voltage appliances or devices.

It can be for simple doorbells, electric stoves, microwaves, etc.

Transformers can be used to raise voltages as well from electric generators to transmit electrical power over long distances.

110KV Power Transformer

Transformers In A Power System

Transformers help and improve safety and efficiency of power systems stepping up (increasing) or stepping down (decreasing) voltages when needed. Transformers are used for both urban and rural areas. One of its important assets is the proper distribution and regulation of electrical power over long distances.

Transformer Principle

Most transformers work on the principle of mutual induction. Mutual induction is an occurrence that happens when the amount of magnetic flux linked with a coil charges an electromotive force that is induced in the nearest coil.

Most transformers are made up of a rectangular iron core.

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What Is Mutual Induction?

Mutual induction refers to the ability of an inductor in one circuit to induce a voltage in or to another circuit. When a transformer has alternating currents flowing in its conductor, magnetic flux will surround the conductor in proportion to the amount of the current.

What Is Magnetic Flux?

Magnetic flux is the measurement of the total magnetic field that passes through a certain or given area. This helps a lot because it describes the effects of the magnetic force on something that is occupying the given area.

The measurement of magnetic flux is associated with the certain area chosen.

What Is A Magnetic Field?

A magnetic field in transformers is considered a medium or vector field that describes the magnetic influence on flowing electricity including electric currents, electric charges, and as well as magnetic materials. Moving charges in a magnetic field will encounter a force that is perpendicular to its velocity and magnetic field.

What Is A Power Transformer?

Now that you know what a transformer is in a bigger picture, understanding power transformers will be much easier. But if there is something that you do not quite comprehend or if there is something you want to clarify after you have read everything, do not hesitate to contact DAELIM’s team of professionals for assistance.

So, what is a power transformer? To begin with, power transformers are electrical instruments that are employed to transmit power from circuit to circuit within the electromagnetic induction.

The power transfer smoothly runs without any frequency variation. In an electronic network, the power transformer’s state is applied to present a number of AC supplies of multiple voltages and compatible values of current from the public electricity support system.

It is also employed to represent the power transformers within the range of 500 kVA (kilovolt-ampere) or more.

As aforementioned, transformers are used to transmit electricity to a certain component of the electrical circuit in the range of distribution circuits and the generator that it is linked to. Power transformers are capable of doing this process, which is why power transformers are commonly utilized in distribution networks as well to adjust voltage levels.

To understand it better, here is another definition with simpler terms: power transformers convert electricity regardless if it is high or low voltage for various purposes.

In power grids, it is considered to be a major component to supply voltage to the lines that are connected. Its destination is to supply voltages to the connect lines that are connected to the power transformer.
This is why power transformers are somewhat similar to distribution transformers but it has its unique characteristics and attributes. When it comes to applications, power transformers are commonly used for transforming power from circuit to circuit without the change or disruption of frequency.

This is the basic definition of most transformers that is applicable to power transformers.Another thing that is worth mentioning is that there are no moving parts in power transformers. So, they are considered as static devices.

Power transformers only operate on an AC supply, but this does not mean that it does not work on multiple principles.

They can also be used for regulating transmission networks that are at high voltages, both stepping up and stepping down. They are commonly generated at above 200 MVA ratings.

Try for free information about the single phase pad mounted distribution transformer

What Is A Power Grid?

Power grid or power distribution grid refers to a network that delivers electricity to its consumers. Power grids involve transmission lines, distribution lines, towers, and generator stations. The generator is responsible for producing energy, converting energy into high voltage levels for distribution.

Parts Of A Power Transformer

Below is where you will find the different functions of power transformers.

Laminated core

The iron core of power transformers is usually laminated with the thin sheet. This prevents the formation of eddy currents in the core, which reduces the chance of loss of energy.

parts of power transformer


Power transformers have two windings which are the primary and secondary winding with different functions. The primary winding is responsible for drawing power from the source while the secondary winding is the coil that delivers energy at the converted voltage to the load.

Insulating materials

Power transformers have solid insulation materials, most transformers have paper press-boards and transformer boards that are formed from cellulose. Cellulose insulation with mineral oil is commonly used as the cooling medium for transformers because of its effectiveness in regulating the transformer’s temperature.

Transformer oil

Transformer oil or mineral is used as the cooling medium for transformers. Some transformers do not use oil but rather use pressurized or forced air as a natural way of cooling down the transformer.

Tap changer

Tap charger is a mechanism in power transformers and other transformers that allows variable turn ratios to be chosen in distinct steps. This process is done by connecting to a number of access points.

Keep reading: A Complete Guide to single-phase overhead conventional transformer

Oil Conservator

Oil conservators in transformers provide enough space for the increase of oil volume inside the transformer when the transformer’s temperature rises. Oil conservators can also be considered as a reservoir for insulating oil.


Breathers help prevent atmospheric moisture from coming in the transformer, which reduces maintenance costs. 

Cooling tubes

Cooling tubes include external radiator tubes that can be from air cooling installed on the transformer tank walls. The fans will suck the air from below and lead it to the top part of the tubes which cools the oil.

If the load is not full, the cooling systems can be operated without the fans.

Buchholz Relay

Buchholz relay is considered a safety device in which it is commonly used for oil-immersed transformers that are usually more than 500 kVA. Its function is to protect the transformer from faults that might occur inside the transformer.
This could be from impulse breakdowns, insulation failures, etc.

Explosion vent

Explosion vents prevent the release of excessive pressure that is generated inside the transformer.

Power Transformer Design

The design of power transformers is composed mostly of metal that is covered by stainless steel sheets. It can be either formed into a shell type or core type.

When it comes to structures, they are connected, and this emplots conductors to produce one-phase transformers or one three-phase transformers.

Three one-phase transformers will require each of its banks isolated from the extra parts, this provides the continuous service if ever one bank refuses to function properly.

For one three-phase transformer, even if it is a core type or shell type, it will simply not function.
This is why three-phase transformers is more const-effective in terms of production. It has lower carbon footprints, which is great for the environment, and better better operations with high efficiency.
Its structure is formed in a fire retardant specific fluid inside a tank. The conservatory on topenables the moving oil to cover it completely. Ther charger of the load drain the side of the tank for the best effective voltage adjustment.

When it comes to its bushings, they enable other parts of the tank to go in and out of the system without disrupting or damaging the outer shell. Power transformers are known to operate beyond low ratings.

Read my article on Types of distribution transformer

Power Transformer Working Principles

A power transformer is a static electrical device that is used to convert a certain value of AC voltage (current) into another or several different values ​​of voltage (current) with the same frequency. When the primary winding is energized with alternating current, an alternating magnetic flux is generated, and the alternating magnetic flux induces an alternating electromotive force in the secondary winding through the magnetic permeability of the iron core. The level of the secondary induced electromotive force is related to the number of turns of the primary and secondary windings, that is, the voltage is proportional to the number of turns.

The main function is to transmit electrical energy, therefore, the rated capacity is its main parameter. The rated capacity is a customary value for expressing power, which is the size of the transmitted electrical energy, expressed in kVA or MVA. When the rated voltage is applied to the transformer, it is used to determine the rated current that does not exceed the temperature rise limit under specified conditions.

The more energy-saving power transformer is the amorphous alloy iron core distribution transformer, and its biggest advantage is that the no-load loss value is extremely low. Whether the no-load loss value can be finally guaranteed is the core issue to be considered in the entire design process. When arranging the product structure, in addition to considering that the amorphous alloy core itself is not affected by external force, the characteristic parameters of the amorphous alloy must be selected accurately and reasonably during the calculation.

10kv class three-phase oil-immersed distribution transformer

Types of Power Transformer

1. Classification of power transformers by function

Power transformers are divided into two categories according to their functions: step-up transformers and step-down transformers. The factory substations all use step-down transformers.

The step-down transformer of the terminal substation, also known as the distribution transformer.

20kv class-three-phase oil-immersed distribution transformer

2. Classification of power transformers by capacity:

ower transformers are divided into two categories according to capacity series, R8 capacity series and R10 capacity series.

The R8 capacity series means that the capacity grade is increased by the multiple of R8≈1.33. The old transformer capacity grades in my country adopt this series, such as: 100kvA, 135kvA, 180kvA, 240kvA, 320kvA, 420kvA, 560kvA, 750kvA, 1000kvA, etc.

R10 capacity series means that the capacity grade is increased by multiples of R10 ≈ 1.26. The capacity level of R10 series is relatively dense, which is convenient for reasonable selection. It is recommended by IEC (International Electrotechnical Commission). my country’s new transformer capacity grades use this series, such as: 100KVA, 125kvA, 160kvA, 200kvA, 250kvA, 315kvA, 400kvA, 500kvA, 630kvA, 800kvA, 1000kvA.

3. Classification of power transformers according to the number of phases:

Power transformers are divided into two categories according to the number of phases, single-phase and three-phase. Factory substations usually use three-phase power transformers.

66kv class-three-phase two winding nltc power transformer

4. Electrical transformers are classified according to the voltage regulation method:

Power transformers are divided into two categories according to the voltage regulation mode, with no-load voltage regulation (also known as non-excitation voltage regulation) and on-load voltage regulation. Most of the factory substations use no-load voltage regulating transformers.

66kv class-three-phase two winding oltc power transformer

5. According to the winding structure electrical transformer:

Power transformers are divided into single-winding autotransformers, double-winding transformers, and three-winding transformers according to the winding structure. Factory substations mostly use double-winding transformers.

110kv class-three-phase three winding nltc power transformer

6. Electrical transformers are classified according to winding insulation and cooling methods:

Power transformers are classified according to winding insulation and cooling methods, including oil-immersed, dry and gas-filled (SF6). Among them, oil-immersed transformers include oil-immersed self-cooling, oil-immersed air-cooled, oil-immersed water-cooled and forced oil circulation cooling. Most of the factory substations use oil-immersed self-cooling transformers.

The so-called gas-filled transformer refers to a transformer in which the magnetic circuit (iron core) and windings of the transformer are located in a casing filled with insulating gas. In the past, SF6 gas was generally used, so it is also called a gas-insulated transformer.

110kv class three-phase three winding oltc power transformer

7. According to the winding conductor material, the electrical transformer is divided into:

Power transformers are divided into two categories according to the winding conductor material, copper winding transformers and aluminum winding transformers. Factory substations mostly used aluminum winding transformers in the past, but low-loss copper winding transformers are now more and more widely used.

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